The issue is caused by one missing permission.

The scoped_search returned an #<ActiveRecord::Relation []>, which is why the subscription list is empty.
The empty object comes from Pool.readable, which is defined in authorization/pool.rb. 


(/opt/theforeman/tfm/root/usr/share/gems/gems/katello-3.0.0.81/app/controllers/katello/api/v2/subscriptions_controller.rb)

-- snip --
    api :GET, "/organizations/:organization_id/subscriptions", N_("List organization subscriptions")
    api :GET, "/activation_keys/:activation_key_id/subscriptions", N_("List an activation key's subscriptions")
    api :GET, "/subscriptions"
    param_group :search, Api::V2::ApiController
    param :organization_id, :number, :desc => N_("Organization ID"), :required => true
    param :host_id, String, :desc => N_("id of a host"), :required => false
    param :activation_key_id, String, :desc => N_("Activation key ID"), :required => false
    param :available_for, String, :desc => N_("Object to show subscriptions available for, either 'host' or 'activation_key'"), :required => false
    param :match_host, :bool, :desc => N_("Ignore subscriptions that are unavailable to the specified host")
    param :match_installed, :bool, :desc => N_("Return subscriptions that match installed products of the specified host")
    param :no_overlap, :bool, :desc => N_("Return subscriptions which do not overlap with a currently-attached subscription")
    def index
      *collection = scoped_search(
        index_relation.uniq, :cp_id, :asc, resource_class: Pool, includes: [:subscription])*

      if params[:activation_key_id]
        key_pools = @activation_key.get_key_pools
        collection[:results] = collection[:results].map do |pool|
          ActivationKeySubscriptionsPresenter.new(pool, key_pools)
        end
      end
      respond(:collection => collection)
    end

    def index_relation
      return for_host if params[:host_id]
      return available_for_activation_key if params[:available_for] == "activation_key"
      *collection = Pool.readable*
      collection = collection.where(:unmapped_guest => false)
      collection = collection.get_for_organization(Organization.find(params[:organization_id])) if params[:organization_id]
      collection = collection.for_activation_key(@activation_key) if params[:activation_key_id]
      collection
    end


-- snip --

Apparently, only the user with the permission :view_subscription (NOT :view_subcriptions) from resource type 'Katello::Subscription' is allowed to read pools. 

(/opt/theforeman/tfm/root/usr/share/gems/gems/katello-3.0.0.81/app/models/katello/authorization/pool.rb)

-- snip --
    module ClassMethods
      def readable
        where(:subscription_id => Katello::Subscription.authorized(:view_subscription))
      end
    end
-- snip --

Problem is there is no such permissions in permissions table.

foreman=# select * from permissions where name like '%view_subscription%';
 id  |        name        |     resource_type     |         created_at         |         updated_at
-----+--------------------+-----------------------+----------------------------+----------------------------
 172 | view_subscriptions | Organization          | 2015-08-28 09:34:19.93697  | 2015-08-28 09:34:19.93697
 (1 rows)

 (note: view_subscriptions != view_subscription)

 User is able to see the subscriptions when the missing permission is added.

 foreman=# insert into permissions (name,resource_type,created_at,updated_at) values ('view_subscription','Katello::Subscription',current_timestamp,current_timestamp);
 INSERT 0 1

 foreman=# select * from permissions where name like '%view_subscription%';
 id  |        name        |     resource_type     |         created_at         |         updated_at
-----+--------------------+-----------------------+----------------------------+----------------------------
 172 | view_subscriptions | Organization          | 2015-08-28 09:34:19.93697  | 2015-08-28 09:34:19.93697
 270 | view_subscription  | Katello::Subscription | 2016-11-03 03:53:40.421009 | 2016-11-03 03:53:40.421009
(2 rows)

This bugzilla is a duplicate of bug 1333219 and has a fix upstream.  I am going to close this bugzilla, but will bump up the priority of the other in hopes that it can get incorporated in to an upcoming 6.2.z.

*** This bug has been marked as a duplicate of bug 1333219 ***